Ebbie, It does look confusing the way it got laid out let me try again. The cross harp is a fourth from the root chord. Or, in a I IV V blues pattern the cross harp key is the second chord you play. Hope that's clearer.

Song Key. Harp Key. A..............D. B..............E. C..............F. D..............G. E..............A.

How do shut up a harp player? Ask him what notes he just played.